U need to figure that once that plants get tall the light goes up and light will be harder to reach the outer plants and lower sides, also a 600 will penetrate the canopy better. If u were doing just 2 rows a 400 would work. The 400watters are great, but there really not all that powerful once the plants get big, there great i mean awsome if u do a lsting, scrog, or 4 plant grow.